Just wondering...are these oil/spirit based paints? Acrylic is water
based isn't it?..

Yes, it is normally water based. Although technically it is possible to have
solvent based acrylic formulations, these would not be described as "quick
drying". Most solvent based paint will be alkyd, or alkyd/acrylic copolymer.
They will also work, but will be more expensive and stink.

Personally for the job in hand, I'd choose

Dulux Trade Mouldshield Fungicidal Quick Drying Eggshell.

This is a water based acrylic formulation with additional fungicide that
might be particularly useful in a shed environment.

http://www.duluxtrade.co.uk/webapp/w...sheets/409.pdf

http://makeashorterlink.com/?V24C35969

Christian.